The invention relates to a method for the detection of contaminants in an optical measuring cuvette of a spectrophotometer, typically an oximeter for determining hemoglobin derivatives, in which measuring cuvette, in addition to at least one sample measurement to obtain a sample spectrum I(λ), at least one reference measurement is performed using a reference liquid to obtain a reference spectrum I0(λ).
Spectrophotometers or spectrometers are generally devices for representing a spectrum and offer the capability of recording and analyzing optical spectra. Spectrometers are used widely, inter alia, in chemical and medical analytics, in order to be able to determine components of a liquid on the basis of their spectral properties. Spectrometers used for this purpose frequently operate according to the polychromator principle, i.e., in this method the incident light is first split using a polychromator into its spectral components, which may thus be imaged simultaneously on a detector array, after the passage through the sample liquid provided in the measuring cuvette. In this way, the entire spectrum can be registered simultaneously (Optical Multichannel Analyzer (OMA) or Multi Channel Spectrometer (MCS)). Modern multichannel spectrometers may transmit a complete spectrum very rapidly to the analysis electronics. A typical field of use for spectrometers is, for example, the analytical determination of hemoglobin derivatives in blood, the so-called CO-oximetry. An example of such a spectrometry module is the COOX module of the cobas b 221 (Roche Diagnostics GmbH, Germany) for the determination of bilirubin (Bili), total hemoglobin (tHb), and the hemoglobin derivatives oxyhemoglobin (O2Hb), desoxyhemoglobin (HHb), carboxyhemoglobin (COHb), and methemoglobin (MetHb). The hemoglobin derivatives and bilirubin are determined by spectrophotometry on the basis of the Lambert-Beer law (see equation (1)). The optical system of this CO-oximetry module essentially comprises a halogen lamp, gap, cuvette holder having a measuring cuvette, polychromator, and detection unit. The light of a halogen lamp is conducted to the cuvette holder with the aid of an optical fiber. In the measuring cuvette, the light is partially absorbed by the sample and partially transmitted. The absorption is characteristic of the composition of the sample. The transmitted light is conducted to the polychromator by a further optical fiber, where it is split into its spectral components and imaged on the surface of a photosensitive receiver (CCD sensor). The absorption and finally the concentration of the hemoglobin derivatives are calculated from the electrical signal resulting therefrom. In order to achieve high reliability in operation, the polychromator is calibrated using an installed spectral light source.
As noted above, the concentration calculation of the hemoglobin derivatives is based on the measured summation absorption of the individual components at multiple wavelengths:Aλ=log (I0λ/Iλ)=ΣεIλ*ci*d   (1)in which:                λ wavelength        i ith individual component        A absorption        I0 reference intensity: intensity of the transmitted light of the cuvette filled with water or air        I sample intensity: intensity of the transmitted light of the cuvette filled with a measuring liquid (e.g., blood)        ε extinction coefficient        c concentration        d layer thickness (internal diameter of the cuvette).        
Both the measuring liquid and also the reference liquid or the reference medium (e.g., non-absorbing liquid) are measured at staggered times in the same cuvette; i.e., the cuvette must be cleaned and subsequently filled with reference liquid between the two measuring points. A disadvantage of the current measuring method lies precisely therein: if the sample and/or measuring liquid is not completely washed out of the cuvette, or an optically interfering layer forms in the measuring cuvette over time—after multiple measuring cycles each having one sample measurement and at least one reference measurement—it remains unknown. The light of the reference measurement is additionally absorbed by such contaminants, i.e., I0 no longer corresponds to the pure reference spectrum (spectrum of the excitation light source, possibly superimposed with spectral effects of optical system (e.g., filter) and by characteristic absorptions of the uncontaminated cuvette filled with reference solution), but rather is additionally superimposed with the absorption of the contaminant. If these errors remain unrecognized, the absorption A of the sample is incorrectly calculated, which inevitably results in incorrect concentration values.
A method for the spectrophotometric determination of the concentration of a number of hemoglobin derivatives in whole blood is known from EP 0 210 417 B1 in this context, in which the turbidity caused by the blood sample is taken into consideration. The blood sample has a number n of individual wavelengths applied thereto, which is at least equal to the number of the hemoglobin derivatives to be determined plus one, the concentrations being determined on the basis of the absorption values at the individual wavelengths by using sets of predetermined coefficients, which represent the absorption characteristics of the individual hemoglobin derivatives at each of the wavelengths and the absorption characteristics of at least one turbidity component at each of the wavelengths. The concentrations of the hemoglobin derivatives are then calculated on the basis of an equation system using an n×n matrix, the turbidity caused by the whole blood is being treated mathematically like one of the concentrations of the hemoglobin derivatives to be calculated. An application of this method to optically interfering layers in the measuring cuvette, such as deposits of prior blood samples, is unrewarding, however, because the resulting concentration values would not be differentiable from the measured values of a current sample measurement.
Furthermore, methods and devices for monitoring contaminant states of various liquids are also known from other technical fields, which are described, for example, in WO 2004/070369 A1. A method is presented here for determining and/or monitoring contaminant states in liquids, in which a white light LED and at least one injection luminescence diode, which emits infrared or ultraviolet radiation, are used. The method exploits the modification of the emission spectrum of the white LED, changes of the peak wavelengths, the ratios of the peaks of the injection luminescence to the peak of the photoluminescence, the selective absorptions, the excitation to fluorescence, the intensity of the peak wavelengths, and the integral emissions, as well as the comparison of the data to modified spectra being used, which are registered with the aid of a fiber-optic compact spectrometer. This method has the disadvantage of the fact that additional light sources are required and the method is not applicable on typical spectral spectrometers without adaptations.